PTFE porous films fabricated by stretching of polytetrafluoroethylene have higher trapping efficiency, for the same level of pressure loss, than do filtration media made from glass fibers, and are therefore particularly favorable for use in high efficiency particulate air (HEPA) filters and ultra low penetration air (ULPA) filters.
While filtration media made from glass fibers have been employed as HEPA filters in the past, a problem in doing so is that filtration media made from glass fibers have high pressure loss. Accordingly, there have been proposed filtration media for air filters employing the aforedescribed PTFE porous films as the filtration medium, in order to achieve less pressure loss than conventional filtration media made from glass fibers.
However, whereas PTFE porous films have low pressure loss, due to the low mechanical strength of PTFE, the film structure is prone to rupture when subjected to external forces, and pressure loss tends to rise as a result. Such external force might include, for example, compressive forces acting on the PTFE porous film in the thickness direction when the PTFE porous film is layered on an air-permeable support material to fabricate a filtration medium.
Typically, in order to ensure a wide filtration medium area, filtration media for air filters are shaped into a zigzag configuration by a pleating process involving the creation of alternating mountain folds and valley folds, and are employed in the form of an air filter unit retained in a frame. In order to ensure channels for air in a filtration medium assembled into an air filter unit, it is necessary to maintain spacing between neighboring mountain fold sections or valley fold sections (pleat spacing). In a conventional air filter unit, with a view to reducing weight and the like, rather than employing separate members such as spacers, it is known to emboss the filtration medium to furnish the medium with a plurality of projections, so that the filtration medium retains the zigzag configuration (for example, see Japanese Laid-open Patent Application No. 2013-52321 and International Patent Application No. 2010/073451). According to the embossed filtration media disclosed in Japanese Laid-open Patent Application No. 2013-52321 and International Patent Application No. 2010/073451, pleat spacing is maintained through contact between embossed projections, which are respectively furnished to two opposing surfaces facing one another across mountain folds or valley folds.